1. Field of the Invention
The present invention relates to a color filter substrate and a display device. More particularly, the present invention relates to a special positional relationship between color filters and spacers that can be used effectively in a high-definition display device and a color filter substrate.
2. Description of the Related Art
A color liquid crystal display (LCD) currently used extensively is provided with color filters for respective picture elements (also called “dots). Typically, three groups of color filters to transmit light rays representing the three primary colors of red (R), green (G) and blue (B), respectively (which will be referred to herein as “R, G and B color filters”) are arranged in a predetermined pattern for those picture elements. One pixel is made up of three picture elements (dots) provided with the R, G and B color filters (which will be referred to herein as “R, G and B picture elements”, respectively), and can contribute to presenting an image in full colors.
In this manner, each set of color filters (or picture elements) typically transmits R, G and B light rays. However, the colors of the color filters may also consist of cyan (C), magenta (M) and yellow (Y) or any other arbitrary combination of three colors. Also, exemplary color filter arrangements include a striped arrangement, a delta arrangement and a mosaic arrangement.
In a normal LCD, a liquid crystal layer is provided between two substrates, and a color filter layer, in which those color filters are arranged for respective picture elements, is provided on one of the two substrates. For example, in a TFT LCD, a liquid crystal layer is often provided between a TFT substrate, including circuit components such as picture element electrodes and TFTs thereon, and a counter substrate, including a counter electrode and the color filter layer thereon. The color filter layer is sometimes provided on the TFT substrate in some known arrangements. However, the majority of LCDs currently on the market includes the color filter layer on their counter substrate. The counter substrate with the color filter layer is often called a “color filter substrate”.
In a normal LCD, spacers are provided to control the thickness of the liquid crystal layer (which is also called a “cell gap”) in the process step of bonding and fixing the color filter substrate and TFT substrate together. However, as the display quality of LCDs has been further improved, the inevitable decrease in the display quality due to the presence of those spacers has become more and more significant.
In a conventional TFT LCD, bead-like or rod-like spacers with a predetermined diameter are dispersed on the surface of the color filter substrate or TFT substrate. For that reason, it is difficult to distribute the spacers at sufficiently uniform densities over the entire display screen, thus possibly making the cell gap non-uniform or causing some display defects due to the agglomeration of the spacers. Also, if the spacers are provided inside of picture elements, then the substantial aperture ratio may decrease or some bright spots may be observed.
To overcome these problems, a method of selectively arranging the spacers in a predetermined region outside of the picture elements (typically in a region that is covered with a black matrix) was developed. For example, a method of providing columnar spacers in a predetermined region by performing a photolithographic process using a photosensitive resin (which is often called a “photoresist”) is used in actual manufacturing processes. In the following description, spacers that are provided at predetermined positions selectively will be referred to herein as “columnar spacers”. But the “columnar” spacers do not have to be exactly columnar but may also be in a wall shape, for example.
However, the present inventors discovered via experiments that the following problems arise when those columnar spacers are used. The problems of the prior art will be described on a conventional TFT LCD as an example.
FIG. 7 is a plan view showing an exemplary arrangement of columnar spacers 2 in a conventional TFT LCD 70. The arrangement shown in FIG. 7 is an arbitrary portion of a display panel. In the accompanying drawings, the location of a picture element 1 will be identified by, the number of a row that is associated with one of the gate lines G1, G2, G3 and so on, and the number of a column that is associated with one of the source lines S1, S2, S3 and so on. Also, in FIGS. 1 through 8, R, G and B represent the colors of color filters that are provided for respective picture elements 1 and the solid circles represent the columnar spacers 2.
Also, when the positional relationship between the picture elements 1 and the columnar spacers 2 is described, each of the columnar spacers 2 will be regarded herein as being associated with (or belonging to) its upper left picture element 1. However, each of those columnar spacers 2 may also be associated with its lower right picture element 1, for example, as long as the same association rule applies to each and every pair of columnar spacer 2 and picture element 1.
If one columnar spacer 2 is provided for each picture element 1 as shown in FIG. 7, then the density of the columnar spacers 2 (i.e., the number of columnar spacers per unit area) will be so high as to apply a shear stress horizontally to the gap between the TFT substrate and the counter substrate. In that case, once one of the TFT and counter substrates has shifted with respect to the other, then the two substrates may not be brought back to their original positions unless some impact is applied. Also, once the relative positions of the two substrates have deviated, deterioration in display quality (e.g., decrease in aperture ratio) will be caused.
On the other hand, as disclosed in Japanese Laid-Open Publication No. 2001-21900, an LCD, in which the columnar spacers are provided for picture elements in a particular color only, is also known. However, if the columnar spacers 2 are provided for only the picture elements 1 in a particular color (e.g., R) as in the LCD 80 shown in FIG. 8, then the density of the columnar spacers 2 will be too low to make the assembly resistible to a high compacting pressure in the process step of bonding the TFT and counter substrates together. As a result, the cell gap may also become non-uniform.
Furthermore, if the width of the black matrix (i.e., the width of the gaps between adjacent picture elements) is minimized to maximize the aperture ratio in a high-definition LCD, then even slight misalignment will affect the display quality seriously. That is to say, once the orientation directions of liquid crystal molecules are disturbed by the columnar spacers on the black matrix, the liquid crystal molecules are also misaligned inside of the picture elements, thus deteriorating the display quality significantly. In the arrangement shown in FIG. 8 in which the columnar spacers 2 are provided for only the picture elements 1 in a particular color, if any misalignment has occurred in the process step of bonding the TFT and counter substrates together, only the picture elements in the particular color will be seriously affected by the disturbance in orientation directions due to the presence of the columnar spacers. As a result, the display quality locally decreases significantly. For example, if the columnar spacers 2 have shifted upper-leftward, then the display quality of the R picture elements will decrease significantly. On the other hand, if the columnar spacers 2 have shifted upper-rightward, then the display quality of the G picture elements will decrease significantly. Accordingly, when the columnar spacers are arranged as shown in FIG. 8, not only decrease in contrast ratio due to the drop of the aperture ratio but also loss of the color balance are brought about.
The problems of the prior art have been described on the striped arrangements shown in FIGS. 7 and 8. However, similar problems also arise in delta arrangements or mosaic arrangements, i.e., no matter how the color filters are arranged.